wonders on every side.

They found the cabin of the strange craft divided into three parts. First came a sort of parlor, with a table and seats arranged on the sides. In the front part of this was a passage leading to the conning tower, or the place for the steersman. Behind the parlor came the sleeping quarters and dining room combined. The bunks were arranged to fold against the wall, and a table in the centre could be shut up when not in use and hoisted to the ceiling, giving plenty of space.

Next came the engine room, and as they entered it the boys could hardly restrain from giving cheers of delight. It was almost filled with machinery, and occupied a little more than half of the whole boat, being twenty-two by ten feet in size.

The two boys did not know the use of one quarter of the machinery and apparatus they gazed on. There were electric motors, storage batteries, two gasoline engines similar to those used in automobiles, pumps, large and small tanks, instruments for measuring the electric current, for telling the temperature, the amount of moisture in the air, the speed of the wind, the speed of the ship, the height to which it went, besides compasses, barometers, telescopes, and other instruments.

There were levers and wheels on every side, switches, valves, electric plugs and handles. Lockers arranged close to the wall and along the floor held supplies and materials. Everything was new and shining, and the professor smiled with pride as he touched piece after piece of machinery, and looked at the different instruments.

"Now we'll go out on the stern," he said.

The boys followed as he ascended the companion steps and emerged on a small platform at the rear end of the cabin.

"Do you know what this is?" asked the professor, touching a long, thin, round object.

"Looks like a gun," replied Mark.

"That's just what it is. It's a machine gun that will fire one hundred shots a minute, and it can be turned in any direction, as it works on a swivel. I don't know that we'll have any use for it, but I thought I'd take it along."

Then the professor pointed out where the propeller shaft ran from the engine room out through the stern, and showed how the rudder was worked by wire ropes extending from it to the conning tower.

"In short we have everything necessary to successfully navigate the air," he went on. "Not a thing has been overlooked. All I have to do is to fill the big bag of oiled silk with a new gas I have discovered and up we go. This is really the most important part of the invention. Without this powerful gas the airship would not rise above the earth.

"But I have found this gas, which can be made in unlimited quantities from simple materials that we can carry with us. The gas has enormous lifting power, and if it was not for that I would not dare make such a large and comfortable airship. As it is, we can sail through the air as easily as if we were on an ocean liner on the sea and much more quickly.

"I generate the gas in the engine room as I need it," the professor went on. "It goes to the oiled silk bag through two tubes. When we have arisen to a sufficient height I start the electric engine, the propeller whirls around, and the ship moves forward, just as a steamboat does when the screw is set in motion. Then all I have to do is to steer."

"It's great!" cried Jack with sparkling eyes.

"It certainly is," agreed Mark.

From the stern the professor took the boys to the conning tower, where there were several wheels and levers, that placed most of the important machines and engines in the boat under the direct control of the steersman. A lever turned one way would send the ship ahead. Turned in the opposite direction it would reverse the course. A wheel like that on an automobile served to direct the rudder and so guided the Monarch's course. Other levers controlled the speed of the engines, and the supply of gas that filled the silk bag.

"Here is where we shall carry our supplies of condensed food," the professor went on, leading the way back into the middle room. "We will take along capsules that will supply us in a small space with meat, vegetables, soups, tea and coffee, besides milk.

"The water we will get as we speed along, dropping down to earth whenever it is necessary. As for clothing, I have an abundant supply."

He opened a locker and disclosed a pile of fur garments. There were big coats, caps and boots, everything made with a furry surface within as well as without.

"Any one would think you were going into some cold country, professor," said Jack, looking at the warm garments.

"So we are! We are going to find the north pole!" exclaimed the old inventor.

"The north pole?" cried Mark.

"That's what I said. Do you boys want to go along in the Monarch to a place where never mortal man has been?"

At that instant there came a loud knock at the door.

CHAPTER V

A PLAN TO SEEK THE NORTH POLE

"Hark! What was that?" exclaimed Professor Henderson in a hoarse whisper.

"Sounded like some one at the door," replied Mark.

"Quick, Washington! Put out the lights! You boys creep back and hide under the bed. My secret must not be discovered now when everything is ready for the trial!"

The boys started back toward the living room, Washington began putting out the lights and then, with the professor, joined the boys. The shed containing the airship was in total darkness, and the negro, turning down the lamp in the cabin, shrouded that in gloom also.

Once more the knock was repeated. It was a peculiar one; first two raps, then a silence, then three blows, followed at intervals by six single raps.

"Who is there?" asked the professor, going close to the door.

"A friend," was the reply.

"Give the countersign."

"The North Pole, and long may it stand!" was the queer answer. It was the same the colored man had given when he sought admission after his second trip to the wreck that afternoon.

Slowly the inventor unfastened the door. As he cautiously opened it a roughly dressed man slipped in.

"What's the need of all this foolishness?" he demanded. "Why have you made it so dark? It's like a pocket. Is any one here?"

The two boys had crawled under the bed before the door was opened, in accordance with the instructions from the old man. The inventor and Washington were the only ones visible in the cabin.

"Why don't you turn up the light?" went on the visitor in fretful tones.
"Are you sure no one is here to learn our secret?"

"Do you see any one?" asked the professor, not wishing to disclose the boys' presence. "Do you think I am so foolish as to waste the labor and toil of years?"

"I didn't think so," said the man, "but as I came along I thought I saw lights in the balloon shed."

"Very likely," admitted Mr. Henderson coolly. "Washington and I were out there doing some work."

"All right," was the rather ungracious answer. "I have those chemicals you wanted."

"Give them to me!" implored the old man in an anxious tone. "I thought you would never bring them."

"Oh, I don't forget so easily. Here you are," and the newcomer passed over a package. "Now when are you going to sail?"

"In about a week," answered the inventor.